Solar cells generally have a finger-type electrode on the surface thereof for the purpose of extracting generated power to the external. It is preferable for the electrode to have a larger width or larger thickness in view of improving the generation efficiency, since the larger the sectional area of the electrode becomes, the more the electrode reduces its resistivity. On the contrary, the electrode prevents sunlight from entering the semiconductor material composing the solar cells, and consequently lowers the generation efficiency. The electrode, typically formed by screen printing using an Ag paste, accounts for 8 to 12% of the total area of the light-receiving surface of the solar cells, and shadowing loss ascribable to this is understood as an obstacle to efforts for improving the generation efficiency.
One known method ever developed in order to improve the generation efficiency through reducing the shadowing loss relates to an OECO (obliquely-evaporated contact) solar cell having a plurality of grooves with any section of rectangular, semicircular and triangular forms fabricated on the light-receiving surface thereof, and having electrodes formed on the individual inner side faces of the grooves only on a single side as viewed along the width-wise direction (“Renewable Energy, Vol. 14, Nos. 1-4, 83-88 (1998), European Patent No. EP0905794A2).
Although the OECO solar cell has a great advantage in terms of energy conversion efficiency, the inventors found out through our investigations that modularization of the OECO solar cell raises various new problems which could have not been encountered before, due to its special design such that the electrodes are formed on the inner side faces of the grooves.
Because the OECO solar cell has the electrodes only on one side face of each groove, the shadowed area will vary depending on the angle of incidence of sunlight, and thus the cell inevitably causes variation in the amount of photo-generated current. This is illustrated in FIG. 2. As seen in FIG. 2, a ratio of shadowed area for an angle of incidence of α is given as B/(A+B+C), but that for an angle of incidence of α′ is given as B′/(A′+B′+C′). This indicates that smaller angle of incidence increases the shadowing loss.
To maximize the generation efficiency of the solar cell module, it is necessary to equalize output current obtainable from the individual solar cells in the module, and thus it is important to equalize the conversion efficiency of the individual solar cells. The OECO solar cell module is, however, not always successful in equalizing the output current from the individual solar cells even if it is configured using solar cells having a uniform basic performance, and even the cells are uniformly irradiated by sunlight. It is because severity of the shadowing loss may differ from cell to cell and the output current may be non-uniform under an oblique incidence of sunlight if no special attention is paid on the direction of attachment of the individual cells in the module.
Considering now that solar cell modules are generally installed on the roof of houses or buildings, and that the OECO solar cell has electrodes only on a single side face of each groove and shows the shadowing loss variable depending on the angle of incidence of sunlight as described in the above, another disadvantage arises that total of daily or annual power output may differ depending on the direction to which the electrode-forming surfaces are directed, and this may prevent performance of the solar cell module from being fully exhibited.
It is therefore a subject of the invention to provide a solar cell module in which output current from the individual solar cells composing the module are equalized even under oblique incidence of sunlight, and a method of effectively installing thus-composed module.